Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique used in radiology to visualize detailed internal structure of a living body. The technique is based on physical principles of nuclear magnetic resonance. Nuclear magnetic resonance, in turn, relies on an atomic property called “spin.” Each proton, electron and neutron in an atom possesses a spin and the net nuclear spin of a particular atomic nucleus is the sum of spins from unpaired protons and neutrons. For example, the nucleus of a hydrogen atom contains one proton and therefore has a net nuclear spin.
A spin can also be thought of as a magnetic moment vector. When atoms with net nuclear spins are placed in an external magnetic field, the magnetic moment vectors of some of the nuclei will precess around the direction of the external field with an orientation that depends on the energy state of the nuclei. This orientation can change as the nuclei absorb and release photons. In a typical sample placed in an external magnetic field, a large number of nuclei are continually transitioning between energy states so that the difference between the number of nuclei in each state produces a net magnetization vector in the sample which points in the direction of the external magnetic field.
The direction of this vector can be altered by applying a radio frequency excitation field with a frequency at which the nuclei resonate while it is in the external magnetic field. When the field is removed, the net magnetization vector returns to its original orientation over time by releasing photons that can be detected as an RF signal. This signal is referred to as the free induction decay (FID) response signal. The time required for the magnetization vector to return to its original orientation is called the relaxation time and varies for different materials. Therefore, different tissues can be distinguished.
The frequency the nuclei resonate at depends on the strength of the applied external magnetic field. The photons released when the RF excitation field is removed have an energy and a frequency that depends on the amount of energy the nuclei absorbed while the excitation field was present. During an MRI scan an additional gradient field is applied to make the external magnetic field strength depend on position, in turn making the frequency of the released photons dependent on position in a predictable manner. Position information can then be recovered from the resulting signal by the use of a Fourier transform.
The excitation RF resonant frequency, also known as the Larmor frequency, is equal to γ·B where B is the magnetic field strength in tesla (T) and γ is a gyromagnetic ratio that is specific to a particular nucleus. Since the human body consists mostly of water molecules that have two hydrogen nuclei or protons, MRI systems use proton spins for imaging. The gyromagnetic ratio for hydrogen is 42.58 MHz/T. For typical MRI systems that use 1.5 T magnetic fields, the RF resonant frequency is approximately 64 MHz. As the field strength of the external magnetic field increases the RF excitation frequency also increases and accordingly, the RF excitation wavelength decreases.
More recently, many MRI systems have been using higher magnetic field strengths, on the order of 3 T. These so-called high-field systems have several advantages including higher picture resolution, faster scans, better signal-to-noise ratios and the ability to visualize physiological processes. Ultra high field strength MRI scanners with field strengths of 4 T or greater are also available.
It would be desirable to further increase the picture resolution, reduce image acquisition time and increase the signal-to-noise ratio in MRI systems.